This invention relates to diamond synthesis.
The synthesis of diamonds using high pressure/high temperature technology has become very well established commercially. This process involves exposing a carbon source to temperatures and pressures in the diamond stable region of the carbon phase diagram in the presence of a suitable catalyst/solvent. Catalysts/solvents useful in diamond synthesis are well known and include metals of Group VIII of the Periodic Table.
While most commercial processes for synthesising diamond produce small or relatively small particles, there are processes known for producing much larger diamonds. These processes generally involve producing the diamond in a reaction vessel in which diamond seed material is separated from a source of substantially pure carbon by a mass of metallic catalyst/solvent such that during synthesis a predetermined temperature gradient between the diamond seed material and the source of carbon is created. The diamond seed material is at a point at which the temperature of the reaction medium will be near the minimum value whilst the source of carbon is placed at a point where the temperature will be near its maximum. A layer of diamond nucleation suppressing material and/or an isolating material is interposed between the mass of metallic catalyst/solvent and the diamond seed material. By way of illustration, reference in this regard may be had to the disclosures of U.S. Pat. Nos. 4,340,576, 4,073,380, 4,034,066, 4,301,134, 3,297,407, 4,322,396 and 4,287,168.
European Patent Publication No. 0290044 describes a method of synthesising large diamond having a diameter of 8 mm or more by the temperature gradient method wherein a (111) or (100) surface of a seed crystal having a diameter of 3 mm or more is used as a growing surface. The entire area of the growing surface is first dissolved in the diamond stable region before crystal growth is started. The crystal growth is effected using a plug of solvent in which the height of the central portion is higher than the height of the peripheral portion. The plug of solvent may contain a certain amount of added carbon.
Japanese Patent Publication No. 2631/1986 describes a process for producing large crystal diamond in a reaction vessel in which a diamond seed is separated from a carbon source by a layer of catalyst/solvent. To minimise seed crystal dissolution, carbon is added to the layer of catalyst/solvent in an amount of 80 to 120 percent of the carbon saturation of the catalyst/solvent under the reaction conditions.